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Swarms are Pokemon that aren't usually found in that game.
In B&W, you can find swarms by checking the board inside the gates from route to route. You can only get swarms after you have the National Pokedex.
Here is the list of swarm in the unova region: http://www.serebii.net/blackwhite/swarms.shtml

A mass outbreak of a certain pokemon. There are certain pokemon which you cannot catch in the game. In white the route gates tell you where a mass outbreak is (which is a swarm). In pearl diamond and platinum dawns little sister tells you. A mass outbreak or swarm lasts for only one day. GO to the route that dawns sister (in platinum) or the route gates (in white) tell you to go. Then when you go there, for that day only you can catch pokemon that you can't normally catch there. For example, in pokemon white if you have a swarm or ourtbreak at route 15 and you go there then you will see wild mankey.